The demand for increasing the packing density of IC packages, multilayer wiring boards and other products of microelectronics is constantly growing and this has caused the need to develop commercially feasible low-dielectric constant ceramics and low-resistance conductors. Metals that have been commonly employed to make metallizing compositions are noble metals such as Au, Au-Pt, Ag-Pt and Ag-Pd, as well as high-melting point base metals such as W, Mo and Mo-Mn. Paste of noble metals such as Au, Au-Pt, Ag-Pt, and Ag-Pd have the advantage that it can be fired in air but, on the other hand, it suffers from the problem of high cost. The high-melting point metals such as W, Mo, and Mo-Mn have the advantage that they can be easily formed in multiple layers since they can be fired simultaneously with ceramics at about 1,600.degree. C. or the temperature at which green ceramic sheets are sintered. Problems with these high melting point metals are low electroconductivity, potential hazard in operations that require firing in a reducing atmosphere, and the need to plate the conductor surface with Ni and other suitable metals to enable soldering. Under these circumstances, use has increased of ceramic wiring boards that employ Cu paste that is inexpensive and which offers high electroconductivity and good solderability.
A problem with this Cu paste is that in the manufacture of ceramic wiring boards, it is difficult to completely combust the organic matter in the paste without causing oxidation of Cu. If the Cu surface is oxidized, not only is the solderability of the paste impaired but its electroconductivity is also reduced. If care is taken to avoid the generation of CuO, the organic binder and other materials will remain without being completely burned away.
Another problem with the use of metallic Cu is that even if the step of removing the binder is separated from the step of Cu firing, the metallic Cu is oxidized to CuO in the step of binder removal and this causes an expansion in volume, leading to various problems such as separation from the substrate.
The present inventors previously proposed in Japanese Patent Application No. 62-254595 a metallizing composition having MnO.sub.2, Pt and Au incorporated in a CuO-Cu system. This metallizing composition was suitable for the purpose of reducing the dielectric constant and expansion coefficient of a ceramic substrate made of crystallizable glass and significant improvement in hermeticity could be achieved when this composition was used in the package of a multi-layered substrate.
However, the present inventor later found that even with this metallizing composition, the size of Cu particles would be a very important factor for the production of a dense film of metallization. The present inventor, therefore, conducted intensive studies in this regard and found that the composition to be described herein is most preferable for the purpose of producing a dense film of metallization. Accordingly, it is an object of the present invention to provide an improvement of the invention proposed in Japanese Patent Application No. 62-254595.